Prior to the present invention, reactions of a dihydric phenol and phosgene to make the corresponding bischloroformate of the dihydric phenol, or polycarbonate, often resulted in "phosgene blow-by", that is the loss of phosgene from the phosgenation reactor condenser vent. Various efforts have been used to eliminate the loss of phosgene into the environment due to its toxic properties. One procedure is to vent the phosgene into a scrubber containing an organic amine, as shown by U.S. Pat. No. 4,493,818. A preferred procedure to minimize phosgene blow-by would be to salvage the phosgene instead of destroying it, in view of economic considerations.
The present invention is based on my discovery that phosgene loss due to blow-by can be reduced by passing the vent stream from the overhead condenser, normally used in batch or continuous reactors, to an absorber fed with an organic solvent, such as methylene chloride followed by the return of the phosgene-containing solvent to the reactor.